Ejector Cycle Heat Recovery Refrigerant Separator

ABSTRACT

A system ( 170; 300; 400 ) comprising a compressor ( 22 ). A heat rejection heat exchanger ( 30; 420 ) is coupled to the compressor to receive refrigerant compressed by the compressor. A separator ( 180 ) has: a vessel ( 181 ); an inlet ( 50 ) coupled to the heat rejection heat exchanger to receive refrigerant; a first outlet ( 54 ) in communication with a headspace of the vessel; and a second outlet ( 52, 52 ′) in communication with a lower portion of the vessel. The system has a heat exchanger ( 182; 220; 220′; 220″; 220 ′″) for transferring heat from refrigerant passing from a heat rejection heat exchanger to liquid refrigerant in the separator.

CROSS-REFERENCE TO RELATED APPLICATION

Benefit is claimed of U.S. Patent Application Ser. No. 61/936,781, filedFeb. 6, 2014, and entitled “Ejector Cycle Heat Recovery RefrigerantSeparator”, the disclosure of which is incorporated by reference hereinin its entirety as if set forth at length.

BACKGROUND

The present disclosure relates to refrigeration. More particularly, itrelates to ejector refrigeration systems.

Early proposals for ejector refrigeration systems are found in U.S. Pat.No. 1,836,318 and U.S. Pat. No. 3,277,660. FIG. 2 shows one basicexample of an ejector refrigeration system 20 drawn from US PatentApplication Publication 2013/0111934 (the '934 publication) thedisclosure of which is incorporated in its entirety herein as is setforth at length. The system includes a compressor 22 having an inlet(suction port) 24 and an outlet (discharge port) 26. The compressor andother system components are positioned along a refrigerant circuit orflowpath 27 and connected via various conduits (lines). A discharge line28 extends from the outlet 26 to the inlet 32 of a heat exchanger (aheat rejection heat exchanger in a normal mode of system operation(e.g., a condenser or gas cooler)) 30. A line 36 extends from the outlet34 of the heat rejection heat exchanger 30 to a primary inlet (liquid orsupercritical or two-phase inlet) 40 of an ejector 38. The ejector 38also has a secondary inlet (saturated or superheated vapor or two-phaseinlet) 42 and an outlet 44. A line 46 extends from the ejector outlet 44to an inlet 50 of a separator 48. The separator has a liquid outlet 52and a gas outlet 54. A suction line 56 extends from the gas outlet 54 tothe compressor suction port 24. The lines 28, 36, 46, 56, and componentstherebetween define a primary loop 60 of the refrigerant circuit 27. Asecondary loop 62 of the refrigerant circuit 27 includes a heatexchanger 64 (in a normal operational mode being a heat absorption heatexchanger (e.g., evaporator)). The evaporator 64 includes an inlet 66and an outlet 68 along the secondary loop 62 and expansion device 70 ispositioned in a line 72 which extends between the separator liquidoutlet 52 and the evaporator inlet 66. An ejector secondary inlet line74 extends from the evaporator outlet 68 to the ejector secondary inlet42.

In the normal mode of operation, gaseous refrigerant is drawn by thecompressor 22 through the suction line 56 and inlet 24 and compressedand discharged from the discharge port 26 into the discharge line 28. Inthe heat rejection heat exchanger, the refrigerant loses/rejects heat toa heat transfer fluid (e.g., fan-forced air or water or other liquid).Cooled refrigerant exits the heat rejection heat exchanger via theoutlet 34 and enters the ejector primary inlet 40 via the line 36.

The exemplary ejector 38 (FIG. 3) is formed as the combination of amotive (primary) nozzle 100 nested within an outer member 102. Theprimary inlet 40 is the inlet to the motive nozzle 100. The outlet 44 isthe outlet of the outer member 102. The primary refrigerant flow 103enters the inlet 40 and then passes into a convergent section 104 of themotive nozzle 100. It then passes through a throat section 106 and anexpansion (divergent) section 108 through an outlet 110 of the motivenozzle 100. The motive nozzle 100 accelerates the flow 103 and decreasesthe pressure of the flow. The secondary inlet 42 forms an inlet of theouter member 102. The pressure reduction caused to the primary flow bythe motive nozzle helps draw the secondary flow 112 into the outermember. The outer member includes a mixer having a convergent section114 and an elongate throat or mixing section 116. The outer member alsohas a divergent section or diffuser 118 downstream of the elongatethroat or mixing section 116. The motive nozzle outlet 110 is positionedwithin the secondary nozzle convergent section 114. As the flow 103exits the outlet 110, it begins to mix with the flow 112 with furthermixing occurring through the mixing section 116 which provides a mixingzone. In operation, the primary flow 103 may typically be supercriticalupon entering the ejector and subcritical upon exiting the motivenozzle. The secondary flow 112 is gaseous (or a mixture of gas with asmaller amount of liquid) upon entering the secondary inlet port 42. Theresulting combined flow 120 is a liquid/vapor mixture and deceleratesand recovers pressure in the diffuser 118 while remaining a mixture.Upon entering the separator, the flow 120 is separated back into theflows 103 and 112. The flow 103 passes as a gas through the compressorsuction line as discussed above. The flow 112 passes as a liquid to theexpansion valve 70. The flow 112 may be expanded by the valve 70 (e.g.,to a low quality (two-phase with small amount of vapor)) and passed tothe evaporator 64. Within the evaporator 64, the refrigerant absorbsheat from a heat transfer fluid (e.g., from a fan-forced air flow orwater or other liquid) and is discharged from the outlet 68 to the line74 as the aforementioned gas.

Use of an ejector serves to recover pressure/work. Work recovered fromthe expansion process is used to compress the gaseous refrigerant priorto entering the compressor. Accordingly, the pressure ratio of thecompressor (and thus the power consumption) may be reduced for a givendesired evaporator pressure. The quality of refrigerant entering theevaporator may also be reduced. Thus, the refrigeration effect per unitmass flow may be increased (relative to the non-ejector system). Thedistribution of fluid entering the evaporator is improved (therebyimproving evaporator performance). Because the evaporator does notdirectly feed the compressor, the evaporator is not required to producesuperheated refrigerant outflow. The use of an ejector cycle may thusallow reduction or elimination of the superheated zone of theevaporator. This may allow the evaporator to operate in a two-phasestate which provides a higher heat transfer performance (e.g.,facilitating reduction in the evaporator size for a given capability).

The exemplary ejector may be a fixed geometry ejector or may be acontrollable ejector. FIG. 2 shows controllability provided by a needlevalve 130 having a needle 132 and an actuator 134. The actuator 134shifts a tip portion 136 of the needle into and out of the throatsection 106 of the motive nozzle 100 to modulate flow through the motivenozzle and, in turn, the ejector overall. Exemplary actuators 134 areelectric (e.g., solenoid or the like). The actuator 134 may be coupledto and controlled by a controller 140 which may receive user inputs froman input device 142 (e.g., switches, keyboard, or the like) and sensors(not shown). The controller 140 may be coupled to the actuator and othercontrollable system components (e.g., valves, the compressor motor, andthe like) via control lines 144 (e.g., hardwired or wirelesscommunication paths). The controller may include one or more:processors; memory (e.g., for storing program information for executionby the processor to perform the operational methods and for storing dataused or generated by the program(s)); and hardware interface devices(e.g., ports) for interfacing with input/output devices and controllablesystem components.

The system features a suction line heat exchanger 92 having a leg 94(heat absorption leg) along the suction line between the separator gasoutlet and the compressor inlet. The leg 94 is in heat exchangerelationship with a leg 96 (heat rejection leg) in the heat rejectionheat exchanger outlet line between the heat rejection heat exchangeroutlet and the ejector primary inlet.

SUMMARY

One aspect of the disclosure involves a system comprising a compressor.A heat rejection heat exchanger is coupled to the compressor to receiverefrigerant compressed by the compressor. A separator has: a vessel; aninlet coupled to the heat rejection heat exchanger to receiverefrigerant; a first outlet in communication with a headspace of thevessel; and a second outlet in communication with a lower portion of thevessel. The system has means for transferring heat from refrigerantpassing from a heat rejection heat exchanger to liquid refrigerant inthe separator.

A further embodiment may additionally and/or alternatively include anexpansion device between the heat rejection heat exchanger and theseparator inlet.

A further embodiment may additionally and/or alternatively include theexpansion device being an ejector having: a primary inlet coupled to theheat rejection heat exchanger to receive refrigerant; a secondary inlet;and an outlet coupled to the separator inlet.

A further embodiment may additionally and/or alternatively include theejector secondary inlet being coupled to receive refrigerant from theseparator second outlet by an additional expansion device and the heatrejection heat exchanger.

A further embodiment may additionally and/or alternatively include theseparator first outlet being coupled to a suction port of thecompressor.

A further embodiment may additionally and/or alternatively include theexpansion device being an expansion valve.

A further embodiment may additionally and/or alternatively include apump coupling the separator second outlet to an inlet of the heatabsorption heat exchanger.

A further embodiment may additionally and/or alternatively include aflowpath through the pump merging with a flowpath through the expansionvalve at a junction upstream of the inlet of the heat absorption heatexchanger.

A further embodiment may additionally and/or alternatively include theseparator first outlet being coupled to the compressor.

A further embodiment may additionally and/or alternatively include theseparator first outlet being coupled to a suction port of thecompressor.

A further embodiment may additionally and/or alternatively include theoutlet being coupled to an interstage of the compressor.

A further embodiment may additionally and/or alternatively include thecompressor being the high pressure stage of a two-stage system.

A further embodiment may additionally and/or alternatively include theseparator being configured to: provide mainly liquid refrigerant to anexpansion device upstream of the heat absorption heat exchanger; andprovide mainly vapor refrigerant to the suction port of the compressor.

A further embodiment may additionally and/or alternatively include therefrigerant comprises at least 50% carbon dioxide, by weight.

Another aspect of the disclosure involves a method for operating thesystem comprising running the compressor in a first mode wherein: therefrigerant is compressed in the compressor; refrigerant received fromthe compressor by the heat rejection heat exchanger rejects heat in theheat rejection heat exchanger to produce initially cooled refrigerant;the initially cooled refrigerant passes through the expansion device; anoutlet flow of refrigerant from the expansion device passes to theseparator to separate said liquid refrigerant from refrigerant vapor;said heat is transferred from said refrigerant passing from the heatrejection heat exchanger to said liquid refrigerant.

Another aspect of the disclosure involves a refrigerant separatorcomprising: a vessel; an inlet; a first outlet in communication with aheadspace of the vessel; a second outlet in communication with a lowerportion of the vessel; and a heat exchanger. The heat exchanger has: aninlet; an outlet; and a portion through the lower portion of the vessel

A further embodiment may additionally and/or alternatively include theheat exchanger having an upstream spiral leg and a downstream straightleg.

The details of one or more embodiments are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description and drawings, and fromthe claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a first ejector refrigeration system.

FIG. 2 is a schematic view of a prior art ejector refrigeration system.

FIG. 3 is an axial sectional view of an ejector

FIG. 4 is a schematic view of a second non-ejector refrigeration system.

FIG. 5 is a schematic view of a third non-ejector refrigeration system.

FIG. 6 is a partially schematic vertical sectional/cutaway view of aheat exchange separator.

FIG. 7 is a partially schematic vertical sectional/cutaway view ofanother heat exchange separator.

FIG. 8 is a partially schematic vertical sectional/cutaway view ofanother heat exchange separator.

FIG. 9 is a partially schematic vertical sectional/cutaway view ofanother heat exchange separator.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

FIG. 1 shows an ejector cycle vapor compression (refrigeration) system170. The system 170 may be made as a modification of the system 20 or ofanother system or as an original manufacture/configuration. In theexemplary embodiment, like components which may be preserved from thesystem 20 are shown with like reference numerals. Operation may besimilar to that of the system 20 except as discussed below with thecontroller 140 controlling operation responsive to inputs from varioustemperature sensors and pressure sensors.

The FIG. 1 embodiment replaces the FIG. 2 separator 48 and suction lineheat exchanger 92 with a combined separator and heat exchanger 180having a vessel 181. The heat exchanger 180 has a conventional maininlet 50 coupled to the line 46 from the ejector outlet 44. Aconventional liquid outlet 52 and vapor outlet 54 are also provided.FIG. 1 further shows a surface 58 of a body of liquid refrigerant in thelower portion of the vessel 181 with vapor in a headspace thereabove.The unit 180, however, is in heat exchange relationship with refrigerantpassing along the line 36 from the outlet 34 of the heat rejection heatexchanger 30 to the primary inlet 40 of the ejector. The heat exchangerportion of 180 is shown as 182 having a leg 184 extending between aninlet 186 and an outlet 188 in heat exchange relation with refrigerantin the unit interior.

In normal operation, refrigerant passing along the primary flowpaththrough line 36 passes into the heat exchanger 182 via inlet 186 andrejects heat to the accumulated refrigerant. A portion of the leg 184(e.g., a lower portion) extends low on the unit 180 to be immersed inliquid refrigerant below the surface 58. This immersion allows thegreatest rejection of heat from the primary flowpath before entering theejector inlet.

Whereas the separator 48 of FIG. 2 or the combined separator and heatexchanger 180 deliver essentially pure vapor from their vapor outlets54, and essentially pure liquid from their liquid outlets 52, the '934publication discloses that it may be desirable to replace one or both ofthese flows with a slightly mixed state flow.

For example, by feeding a two-phase mixture into the compressor, thedischarge temperature of the compressor can be reduced if desired (thusextending the compressor system operating range). Feeding a suction lineheat exchanger (SLHX) and/or compressor with small amount liquid arealso expected to improve both SLHX and compressor efficiency. Exemplaryrefrigerant is delivered as 85-99% quality (vapor mass flow percentage),more narrowly, 90-98% or 94-98%. The power required for compression of avapor increases which increased suction enthalpy. For hermeticcompressors the refrigerant vapor is used to cool the motor. Forexample, in many compressors, the suction flow is first passed over themotor before entering the compression chamber (raising the temperatureof refrigerant reaching the compression chamber). By supplying a smallamount of liquid in the vapor of the suction flow, the motor can becooled while reducing the temperature increase of the refrigerant as itpasses over the motor. Furthermore, some compressors are tolerant ofsmall amounts of liquid entering the suction chamber. If the compressionprocess is begun with some liquid, the refrigerant will remain coolerthan it otherwise would, and less power is required for the compressionprocess. This is especially beneficial with refrigerants that exhibit alarge degree of heating during compression, such as CO₂. The negativeside of providing liquid refrigerant to the compressor is that theliquid is no longer available for producing cooling in the evaporator64. The optimum choice of quality provided to line 56 is determined bythe specific characteristics of the system to balance theseconsiderations.

A small amount of liquid refrigerant can also be used to improve theperformance of a SLHX. SLHXs are typically of counter-flow design. Thetotal heat transfer is limited by the fluid side that has the minimumproduct of flow rate and specific heat. For a refrigeration system SLHXwith pure vapor on the cold side and pure liquid on the hot side, thecold-side vapor is limiting. However, a small amount of liquid providedto the cold-side effectively increases its specific heat. Thus more heatmay be transferred from the same SLHX, or conversely, for the same heattransfer a smaller heat exchanger may be used if a small amount ofliquid is added to the vapor.

Also by feeding a two-phase mixture to the expansion valve upstream ofthe evaporator one can precisely control the system capacity, which canprevent unnecessary system shutdowns (comfort and improved reliability)and improve temperature control. This may help improve refrigerantdistribution in the evaporator manifold and further improve evaporatorperformance Exemplary refrigerant is delivered as 1-10% quality (vapormass flow percentage), more narrowly 2-6%. Direct expansion evaporatorstypically have poor heat transfer in the very low and very high qualityranges. For these evaporator designs providing higher quality mayimprove the heat transfer coefficient at the entrance region of theevaporator (where quality is the lowest).

Thus, the separator/heat exchanger 180 may have means for providing atleast one of the 1-10% quality refrigerant to the heat absorption heatexchanger and the 90-99% quality refrigerant to at least one of thecompressor and, at present, a suction line heat exchanger.

Examples of such means involving configuration of tubes and their inletsis disclosed in the '934 publication.

The controller may control an operation in response to input from aplurality of sensors such as temperature sensors and pressure sensors. Afirst exemplary pair of these sensors 600 (self heat sensor) and 602(regular sensor) is shown in the suction line 56 between the outlet 186and the suction port 24 of FIG. 1. A second exemplary pair 604, 606 isshown along the line 74 downstream of the evaporator and upstream of theejector secondary inlet in FIG. 1. An alternative method is to use themeasured discharge superheat and, through known calibration of thecompressor isotropic efficiency, have the controller determine thesuction quality condition. This may be determined via a dischargesuperheat sensor 610 in the discharge line at the exit of thecompressor. This may be a relatively cost effective method for measuringthe quality of refrigerant discharged from the outlet 186. A thirdvariation involves a superheat sensor 614 (FIG. 1) within the compressordownstream of the motor.

FIG. 4 shows use of the separator/heat exchanger 180 in an ejector-lesssystem 300. An expansion device 330 (e.g., similar to the expansiondevice 70) replaces the ejector and has an inlet along the line 36downstream of the heat exchanger 182 (e.g., the heat exchanger outlet188 is coupled to the inlet of the expansion device 330 via anappropriate conduit). The outlet of the expansion device 330 feeds theinlet 66 of the heat rejection heat exchanger 64 flow from the outlet 68of the heat rejection heat exchanger 64 passes to the separator/heatexchanger inlet 50. Liquid refrigerant from the outlet 52 is passed tothe inlet 66 of the heat rejection heat exchanger via a conduit 310defining a flowpath extending to a junction 312 with the line 36 and itsflowpath. A pump 320 having an inlet 322 and an outlet 324 is locatedalong the lines or conduit 310 so as to pump the liquid refrigerant tocreate an open loop flow via the line 310, through the heat rejectionheat exchanger 64, and returning to the separator inlet 50. Theexemplary pump 320 is a centrifugal pump driven by an electric motor.

Operation of the pump 320 and expansion valve 330 may be under thecontrol of the controller 140. For example, expansion valve 330 may bean electronic expansion valve (EXV) or may be a thermal expansion valve(TXV) controlled by superheat at inlet port of compressor at pipe 56.Pump 320 may be controlled in response to superheat of inlet port ofcompressor at pipe 56 or refrigerant liquid level 58 in the phaseseparator. For example, as long as superheat is less than a thresholdsuch as 0.5° C., or refrigerant liquid level is at least at a thresholdsuch as ¾ of the separator height, the controller will run the pump topump refrigerant liquid back to the evaporator. A check valve 326downstream of the pump serves to prevent refrigerant flow back to thepump.

FIG. 5 shows a second ejector-less system 400 utilizing theseparator/heat exchanger 180. There is a two-stage compressor 22 havingstages 22A and 22B. This may alternatively represent two separatecompressors 22A and 22B. The discharge port 26B of the second stageconnects to a discharge line to in turn feed a heat exchanger 420 beforeentering the heat exchanger 182 and feeding back into the inlet 50 ofthe separator/heat exchanger 180. In the exemplary implementation, anexpansion device 430 is in the line between the heat exchanger 182 andthe inlet 150. The exemplary expansion device 430 is a high pressureexpansion valve such as an EXV. The high pressure expansion valve servesto convert supercritical refrigerant (e.g., CO₂) to a two-phase state.

The refrigerant from the liquid outlet 52 passes through the expansiondevice 70 and the heat rejection heat exchanger 64 to return to theinlet 24A of the low pressure compressor or stage 22A. A vapor line fromthe outlet line 54 may extend to the inlet 24B of the high pressurecompressor or stage 22B.

FIG. 5 shows an economizer valve 440 (allowing an economizer mode whenopen) and a one-way check valve 442 located between the outlet 54 andthe inlet 24B to prevent reverse low pressure flow back into theseparator/heat exchanger through the outlet 54. The outlet 26A of thefirst compressor or stage 22A is connected to a heat exchanger 450. Theexemplary heat exchangers 420 and 450 are refrigerant-air heatexchangers integrated in a unit 452 where a fan (not shown) drives anairflow across the heat exchanger 420 then the heat exchanger 450 so asto reject heat to the environment. 420 is upstream along the airflowbecause it is desirable that this receive the coldest air to determinedownstream conditions along the refrigerant flowpath. Alternativeconfigurations may involve separate airflows across the two heatexchangers 420 and 450.

A line from the outlet of the heat exchanger 450 extends back to asuction location of the high pressure compressor or stage 22B. Thus, insome operational modes, flows may merge from the outlet 54 and the firststage to feed the second stage. FIG. 5 also shows a bypass 460 betweenthe suction location of the first compressor and the suction location ofthe second compressor. The bypass line through which flow is controlledby a valve 470. The exemplary valve 470 is an unload bypass valve and isused to bypass refrigerant around the first stage compressor 22A whenthe loading requirement is low (and the first stage is shut off).

FIG. 6 shows one example of the heat exchanger as a twisted spiral tubeheat exchanger 220 having an upstream spiral leg 222 extending downwardand a downstream straight leg 224 extending upward within the spiral.

FIG. 6 also shows various optional variations on the basic separatorstructure. The separator has an inlet tube 230 extending to an outletend 232 to deliver refrigerant toward an interior sidewall surface ofthe vessel to be deflected with liquid descending into the accumulationin the lower portion 59 and thus avoid/limit foaming. Also well up inthe headspace (shown even higher than the outlet end 232 is the inletend 242 of an outlet conduit 240. the exemplary outlet conduit 240 is aJ-tube having a lower end portion or turn 244 near the bottom of thevessel. near the bottom of the lower end 244, the conduit includes anaperture or orifice 246 which serves as an oil pickup to entrain oilinto vapor flow through the conduit 240. In an exemplary embodiment, anupper extreme of the orifice is below a lower extreme of the outlet 52so as to keep a level of any oil accumulation below the outlet 52 tolimit/prevent oil flow out the outlet 52.

An exemplary spacing of the outlet lower end above the orifice upper endis at least 2 mm (e.g., 2 mm to 10 mm, or at least 5 mm). In anexemplary embodiment, a lower extreme of the heat exchanger is above anupper extreme of the outlet 52 so as to keep the surface level 58 ofliquid refrigerant sufficiently above the outlet 52 to limit/preventvapor flow out the outlet 52 (e.g., the heat exchanger will not be ableto boil off refrigerant below its lower end). An exemplary spacing ofthe heat exchanger lower end above the outlet upper end is at least 5 mm(e.g., 5 mm to 20 mm, or at least 10 mm while still in a lower half orthird or quarter or fifth of the vessel interior height). An alternativeoutlet location 52′ at the bottom of the vessel is shown in brokenlines.

FIG. 7 shows an alternative variation otherwise similar to FIG. 6 butwhere the downstream leg 224′ of the heat exchanger 220′ is aside ratherthan within the spiral upstream leg 222′.

FIG. 8 shows an alternative variation otherwise similar to FIG. 6 butwhere the upstream leg 222″ and downstream leg 224″ of the heatexchanger 220″ are legs of a U-tube and the tube is finned to enhanceheat transfer. Fins may be plate fins or one or more helical fins.

FIG. 9 shows an alternative variation otherwise similar to FIG. 8 butwhere the U portion of the heat exchanger 220″' tube along its legs222″, 224″' and base has a heli-enhanced deformed sidewall (e.g., doublehelix outward deformation shown).

The system may be fabricated from conventional components usingconventional techniques appropriate for the particular intended uses.

Although an embodiment is described above in detail, such description isnot intended for limiting the scope of the present disclosure. It willbe understood that various modifications may be made without departingfrom the spirit and scope of the disclosure. For example, whenimplemented in the remanufacturing of an existing system or thereengineering of an existing system configuration, details of theexisting configuration may influence or dictate details of anyparticular implementation. Accordingly, other embodiments are within thescope of the following claims.

1. A system (170; 300; 400) comprising: a compressor (22); a heatrejection heat exchanger (30; 420) coupled to the compressor to receiverefrigerant compressed by the compressor; a separator (180) having: avessel (181); an inlet (50) coupled to the heat rejection heat exchangerto receive refrigerant; a first outlet (54) in communication with aheadspace of the vessel; a second outlet (52, 52′) in communication witha lower portion of the vessel; a heat absorption heat exchanger (64);and means (182; 220; 220′; 220″; 220″') for transferring heat fromrefrigerant passing from the heat rejection heat exchanger to liquidrefrigerant in the separator.
 2. The system of claim 1 furthercomprising: an expansion device (38; 330; 430) between the heatrejection heat exchanger and the separator inlet.
 3. The system of claim2 wherein the expansion device is: an ejector (38) having: a primaryinlet (40) coupled to the heat rejection heat exchanger to receiverefrigerant; a secondary inlet (42); and an outlet (44) coupled to theseparator inlet.
 4. The system of claim 3 wherein the ejector secondaryinlet is coupled to receive refrigerant from the separator second outletby an additional expansion device (70) and the heat rejection heatexchanger.
 5. The system of claim 3 wherein the separator first outletis coupled to a suction port (24) of the compressor.
 6. The system ofclaim 2 wherein the expansion device is: an expansion valve (330; 430).7. The system of claim 6 further comprising: a pump (320) coupling theseparator second outlet to an inlet (66) of the heat absorption heatexchanger.
 8. The system of claim 7 wherein: a flowpath through the pumpmerges with a flowpath through the expansion valve at a junction (312)upstream of the inlet (66) of the heat absorption heat exchanger.
 9. Thesystem of claim 1 wherein the separator first outlet is coupled to thecompressor.
 10. The system of claim 9 wherein the separator first outletis coupled to a suction port (24) of the compressor.
 11. The system ofclaim 9 wherein the outlet is coupled to an interstage of thecompressor.
 12. The system of claim 9 wherein the compressor is the highpressure stage (22B) of a two-stage system.
 13. The system of claim 9wherein the separator is configured to: provide mainly liquidrefrigerant to an expansion device upstream of the heat absorption heatexchanger; and provide mainly vapor refrigerant to the suction port ofthe compressor.
 14. The system of claim 1 wherein: refrigerant comprisesat least 50% carbon dioxide, by weight.
 15. A method for operating thesystem of claim 1 comprising running the compressor in a first modewherein: the refrigerant is compressed in the compressor; refrigerantreceived from the compressor by the heat rejection heat exchangerrejects heat in the heat rejection heat exchanger to produce initiallycooled refrigerant; the initially cooled refrigerant passes through theexpansion device; an outlet flow of refrigerant from the expansiondevice passes to the separator to separate said liquid refrigerant fromrefrigerant vapor; said heat is transferred from said refrigerantpassing from the heat rejection heat exchanger to said liquidrefrigerant.
 16. A refrigerant separator comprising: a vessel (181); aninlet (50); a first outlet (54) in communication with a headspace of thevessel; a second outlet (52;52′) in communication with a lower portionof the vessel; and a heat exchanger (182) having: an inlet (186); anoutlet (188); and a portion through the lower portion of the vessel. 17.The system of claim 16 wherein the heat exchanger comprises: an upstreamspiral leg and a downstream straight leg.
 18. A system comprising therefrigerant separator of claim 16 and further comprising: a compressor(22); a heat rejection heat exchanger (30; 420) coupled to thecompressor to receive refrigerant compressed by the compressor; and anejector (38) having: a primary inlet (40) coupled to the heat rejectionheat exchanger to receive refrigerant; a secondary inlet (42); and anoutlet (44) coupled to the separator inlet.
 19. The system of claim 1,wherein the means comprises a heat exchanger (182) having: an inlet(186); an outlet (188); and a portion through the lower portion of thevessel.
 20. The system of claim 19 the heat exchanger comprises: anupstream spiral leg and a downstream straight leg.